Sewer Gas Smell From Shower Drain But P-Trap Is Full

[Art-Emel]

I smell sewer gas every other day or so in my basement bathroom which has a sink, toilet, and shower. The p-trap on all of them are full and when I get down and smell the drain of the shower, it has a slight stench of the sewer.

Is something wrong? How might the gas be getting into the room if the p-trap is full?

My home was built in the 60s and has cast iron drains under the house. The angle of the shower p-trap is too sharp to scope but I had a plumber scope the drain of the toilet next to it and he found no evidence of a leak or larger issue. Water from the toilet and shower seemed to flow easily. But we technically could not see precisely under the shower.

The plumber did replace the wax seal on the toilet and mentioned the seal there is still not the best but when I lean down there, I don't smell a thing.

He also scoped the sewer vents on top of my house and said they were clear.

When I cover the shower drain with a plastic lid, the smell goes away and does not come back so I am pretty sure it is the culprit - but I am trying to keep an open mind as this does not make a lot of sense to the plumber.

I filled the shower drain with vinegar and then got no smell the next day. Then I added baking soda which foamed and the room filled with sewer gas - I assume because the foaming allowed the gas to flow into the house. Does this mean anything?

My only other thing to add is my Radon mitigation. There is a crawlspace near the bathroom, about 12 feet away which has a large plastic tarp over all the dirt and a radon system pulling air up and out. Any chance this is creating enough pressure to occasionally pull sewer gas out of my shower? It sounds silly but I have no idea what it could be.

 

[Twowaxhack]

There might be water leaking out under the shower base and pooling.

 

[Art-Emel]

There might be water leaking out under the shower base and pooling.

I suppose it is possible. I NEVER use the shower. It's been over a year since anyone has used it. I just add water as the ptrap gets low. So if water was leaking under the shower, I am not sure of the source of water, let alone sewage/sewer gas. We scoped the toilet next to it so the source would have to be something else.

 

[Art-Emel]

I suppose it is possible. I NEVER use the shower. It's been over a year since anyone has used it. I just add water as the ptrap gets low. So if water was leaking under the shower, I am not sure of the source of water, let alone sewage/sewer gas. We scoped the toilet next to it so the source would have to be something else.

The other confusing thing, it only smells occasionally. Like it comes in waves. If sewage was under there, I'd think it would be constant.

 

[Twowaxhack]

Sounds like your trap seal is being compromised.

Intermittent clog drain can siphon the trap or it could have a small hole allowing it to leak down just a little, enough to break the seal.

 

[frontporch]

Although I don’t know if they make the correct size, there are plumbing plugs like rubber corks in big box stores for testing plumbing drain lines and also swimming pool supply stores for winterizing. Why not temporarily seal it?

 

[Bird Doo Head]

Wild Guess:
Could gunk be collected on the walls of the drain pipe? We had a seldom used sink that would very occasionally send odor into the room, despite the trap's having plenty of water.
Enzymes ate up the gunk & kept it gone since all the bacteria laden residual soap scum & mold were eaten by the enzymes. Hardware stores sell septic tank enzymes that will work.

Mix them in barely warm water and dump in, coating the sides of the pipe. The enzymes will cling. If you can leave it for a couple of days, more will be eaten. Flushing is optional because the odor causing bacteria has been digested & released as carbon dioxide.

I Hate To Ask, But-
Is there any chance that there is water trapped under the shower, either from a seal or caulking leak or from ground water pushing through the concrete? That could cause an occasional odor.

Paul

PS: When you go shopping, what FrontPorch mentioned are called "Test Plugs". They come in all pipe sizes and have a wing nut that pulls up a plate to expand a rubber seal against the pipe's wall. Sample photo is attached. You might have to cut notches in the top plate to fit around screw bosses for the strainer in the shower drain.

 

[frontporch]

The pool plugs are like a cork and can plug a variet of sizes the deeper you insert. They also have a wing nut to make the plug widen and form a better seal. These aren’t expected to hold pressure like the other plugs but should also work well for what you are doing.

This might rule out the pipe as the odor source and relieve you from having to fill the trap in the future

 

[Art-Emel]

I cleaned the drain sidewalls with vinegar and baking soda and a scrubber brush and flushed lots of water through it. The smell has not returned. I am shocked but this seems to have solved it!

 

[Twowaxhack]

I cleaned the drain sidewalls with vinegar and baking soda and a scrubber brush and flushed lots of water through it. The smell has not returned. I am shocked but this seems to have solved it!

I thought you already did that. Weird that a shower drain that’a not used much but maintained the trap would create a stink. Especially with the proper distance from the drain to the trap being observed.

I could understand if the total pipe before the ptrap was significant.

Some people don’t understand why long distances between the fixture and the trap matter. Stink is more likely to form the more pipe you have between trap and fixture outlet.

 

[Art-Emel]

My best guess as to what may have happened is there was an issue with the city where a small amount of water backed up into the shower months ago but then drained back out. It only ever happened once and has been scoped to make sure there are no flow issues. But it may have left sewer residue or something in the p-trap and drain walls which kept re-creating the smell even after being flushed out. After cleaning it, the smell has been gone over a week where it was recurring every 24-48 hours before.

 

[Art-Emel]

I thought you already did that. Weird that a shower drain that’a not used much but maintained the trap would create a stink. Especially with the proper distance from the drain to the trap being observed.

I could understand if the total pipe before the ptrap was significant.

Some people don’t understand why long distances between the fixture and the trap matter. Stink is more likely to form the more pipe you have between trap and fixture outlet.

I did try cleaning it right before I posted my original question but assumed it would not change anything for the same reasons you mentioned above. It really doesn't make a lot of sense. But after waiting over a week, it seems that it did fix it.